A large number of the plastic containers are currently in use, especially for example, plastic bottles and the like. The plastic containers can be produced in a stretch blow molding process. In this method a so-called preform, which can have a tubular shape, on its one longitudinal end has a bottom and on another longitudinal end, a neck region with molded threaded sections or the like, is inserted into the mold cavity of a blow mold and is inflated by a medium which is blown in with overpressure. In doing so the preform can be additionally stretched in the axial direction with a stretching mandrel which is inserted through the neck opening. After this stretching/blowing process, the finished, stretch-set plastic container is removed from the blow mold.
The known preforms can be produced in an injection molding method. Relatively thin-liquid materials can be used due to the injection molding method. A raw material which is often used for producing plastic containers in the stretch blow molding process is PET (polyethylene terephthalate). PET has low viscosity which is desirable for the injection molding process at high temperatures around 280° C. and has been repeatedly tested and its properties have been known for a long time. Unfortunately PET bottles can have extremely poor barrier properties relative to water and light, and the introduction of barrier layers can be complex.
For polyolefins, generally PE or PP, processing via the extrusion process without a stretch blow molding process is a known processing method for bottles. However, polyolefins do not have optimum stretch setting in the subsequent stretch blow molding process. So that PET containers achieve the required mechanical strengths (buckling pressure, internal compressive strength, etc.), they should not fall below a minimum wall thickness of roughly 0.2 mm. For ecological reasons and as a result of high raw material costs, there is on the other hand great interest in still further reducing the wall thicknesses of the plastic containers.
Preforms for plastic containers which have been produced in the stretch blow molding process can have a multilayer structure. For example, preforms are known with three or more layers which include, for example, barrier layers or getter layers for gases, opaque layers, etc. Multilayer preforms are also known in which one or more layers contain regenerate material. To join the different layers to one another, a coupling agent as a blend or as a copolymer is admixed, for example, with the base polymer or the barrier layer. The coupling agent can be integrated more or less homogeneously into the raw material or barrier material. It can develop its action however only on the interfaces of the layers which are to be joined to one another. In the remaining raw material, it may be of no benefit and in part can degrade the mechanical or barrier properties there. For ecological reasons and for reasons of costs there is however interest in reducing the amount of the coupling agent.
In the production of the preforms in the injection molding process, known pressures of up to roughly 1000 bar can occur in the melt. In order to keep the closing forces within reasonable boundaries, the injection molds often have only a few cavities. This can lead to relatively high production costs in preforms with large neck diameters of 48 mm and more for so-called wide-neck bottles with relatively small capacities.
EP-1 344 618 A1 discloses preforms for use in a stretch blow molding process. The preform, whose production is not detailed, has a shoulder region which is arched to the outside following the supporting ring which separates the neck section of the preform from its body section. The inner surface of the preform in the region of the supporting ring is made as a conical surface which widens in the direction of the spherical bottom of the preform. EP-1 344 618 A1 discloses the forming of the preform with a body section which has distinctly larger inside and outside diameters than the neck section. This known preform in its production is already so strongly deformed that a further and prompt axial and radial orientation in the following stretch blow molding process is too little and does not lead to the desired stretch setting of the plastic container which has been produced.